This invention relates generally to vehicle rear vision systems and, in particular, to a rear vision system made up of optical elements.
In U.S. Pat. Nos. 5,594,593 and 5,617,245 issued to Peter J. Milner, the disclosures of which are hereby incorporated herein by reference, an optical rearview unit for a vehicle is disclosed which substantially reduces protrusions beyond the exterior surface of the vehicle, as compared with existing exterior rearview mirrors, while providing with rearward and lateral fields of view of the vehicle. The optical system spans the body line of the vehicle so that only the outer surface of an objective refractor lies outside the body line. This minimizes the projection of the rearview system laterally of the vehicle and, at the same time, brings the image position inboard so that the observer does not have to divert his, or her, line of sight so far from the straight-ahead position in order to view the image presented by the rearview system as is currently the case with conventional exterior door-mounted rearview mirrors. This is accomplished in the Milner patents by a first refractor optical element which is positioned in the path of light from objects to the rear of the vehicle passing through the refractor element to a reflector member from which the light is reflected towards the driver. A second refractor element is provided for compensating chromatic aberrations introduced into the viewing system by the first refractor element.
While the Milner optical rearview system provides significant advantages over conventional exterior rearview mirrors, several problems have precluded successful commercial utilization of the Milner mirror. In one embodiment of the Milner mirror, one, or both, refractor element is a wedge. The difficulty with the use of the wedge optical element is the production of secondary reflections, such as objects within the passenger compartment which are reflected back into the eyes of the driver. Various attempts have been made at overcoming such secondary reflections including the application of anti-reflection coatings to the optical element. However, even anti-reflective coatings have proven inadequate to satisfactorily overcome such secondary reflections. One possible improvement to overcoming secondary reflections is to replace the wedge with a Fresnel prism. The Fresnel prism effectively divides the wedge into a series of individual prismatic mini-elements. In addition to providing an improvement to secondary reflections, the Fresnel prism is advantageous because it may be made from a plastic material rather than glass and provides more flexibility in system design thereby reducing interference with vehicle body configuration.
However, the Fresnel prism itself introduces additional undesirable consequences in the design of an optical rearview system. One such undesirable consequence is the production of "flare" which is a spread or smear of light superimposed over a normal image especially in high contrast conditions such as those occurring at night. Such smear of light is believed to originate from a location outside of the field of view of the rearview system, such as from the side of the vehicle. It is believed that flare results from light sources outside of the field of view of the system contacting the draft surfaces defined by the transition from one Fresnel prismatic mini-element to the next mini-element. Because the eyes of the driver tend to be adapted to low lighting conditions, which is the condition in which such flare often occurs, the presence of an aberration in the form of a spread, or smear, of light superimposed over the normal image is exceptionally undesirable and annoying.